Polyamide-based film, preparation method thereof, cover window and display device comprising the same

ABSTRACT

The embodiments relate to a polyamide-based film that is excellent in optical properties and mechanical properties, to a process for preparing the same, and to a cover window and a display device comprising the same. There are provided a polyamide-based film, which comprises a polyamide-based polymer and has a plasticity characteristic value represented by the following equation of 18.45 to 19.97%, a process for preparing the same, and a cover window and a display device comprising the same: Plasticity characteristic value (%)=(100−nIT)/(HIT/HM). In this equation, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm2), and HM is the composite hardness (N/mm2).

The present application claims priority of Korean patent application number KR 10-2021-0031079 filed on Mar. 9, 2021. The disclosure of each of the foregoing applications is incorporated herein in their entirety.

TECHNICAL FIELD

Embodiments relate to a polyamide-based film, to a process for preparing the same, and to a cover window and a display device comprising the same.

BACKGROUND ART

Polyamide-based polymers are excellent in resistance to friction, heat, and chemicals. Thus, they are employed in such applications as primary electrical insulation, coatings, adhesives, resins for extrusion, heat-resistant paintings, heat-resistant boards, heat-resistant adhesives, heat-resistant fibers, and heat-resistant films.

Polyamide is used in various fields. For example, polyamide is made in the form of a powder and used as a coating for a metal or a magnetic wire. It is mixed with other additives depending on the application thereof. In addition, polyamide is used together with a fluoropolymer as a painter for decoration and corrosion prevention. It also plays a role of bonding a fluoropolymer to a metal substrate. In addition, polyamide is used to coat kitchenware, used as a membrane for gas separation by virtue of its heat resistance and chemical resistance, and used in natural gas wells for filtration of such contaminants as carbon dioxide, hydrogen sulfide, and impurities.

In recent years, polyamide has been developed in the form of a film, which is less expensive and has excellent optical, mechanical, and thermal characteristics. Such a polyamide-based film may be applied to display materials for organic light-emitting diodes (OLEDs) or liquid crystal displays (LCDs), and the like, and to antireflection films, compensation films, and retardation films if retardation properties are implemented.

When such a polyamide-based film is applied to a foldable display, a flexible display, and the like, optical properties such as transparency and colorlessness and mechanical properties such as flexibility and hardness are required. In general, however, since optical properties and mechanical properties are in a trade-off relationship, an improvement in the mechanical properties would impair the optical properties.

Accordingly, research on polyamide-based films with improved mechanical properties and optical properties is continuously required.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The embodiments aim to provide a polyamide-based film that is excellent in optical properties and mechanical properties and a cover window and a display device comprising the same.

The embodiments aim to provide a process for preparing a polyamide-based film that is excellent in optical properties and mechanical properties.

Solution to the Problem

The polyamide-based film according to the embodiments comprises a polyamide-based polymer, wherein the plasticity characteristic value represented by the following Equation 1 is 18.45 to 19.97%.

Plasticity characteristic value (%)=(100−nIT)/(HIT/HM)  [Equation 1]

In Equation 1, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm²), and HM is the composite hardness (N/mm²).

The polyamide-based film according to the embodiments comprises a polyamide-based polymer, wherein the elasticity characteristic value represented by the following Equation 2 is 67 to 69.5%.

Elasticity characteristic value (%)=nIT/{EIT/(EIT+HIT)}  [Equation 2]

In Equation 2, nIT, EIT, and HIT are measured according to the ISO 14577 standard through a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).

The polyamide-based film according to the embodiments comprises a polyamide-based polymer, wherein the second plasticity characteristic value represented by the following Equation 3 is 384 to 416%.

Second plasticity characteristic value (%)=(1−nIT)/{HIT/(EIT+HIT)}  [Equation 3]

In Equation 3, nIT, EIT, and HIT are measured according to the ISO 14577 standard through a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).

The process for preparing a polyamide-based film according to the embodiments comprises polymerizing a diamine compound and a dicarbonyl compound in an organic solvent to prepare a solution comprising a polyamide-based polymer; casting the polymer solution to prepare a gel sheet; and thermally treating the gel sheet.

The cover window for a display device according to the embodiments comprises a polyamide-based film and a functional layer, wherein the polyamide-based film has a plasticity characteristic value represented by the above Equation 1 of 18.45 to 19.97%.

The display device according to an embodiment comprises a display unit; and a cover window disposed on the display unit, wherein the cover window comprises a polyamide-based film and a functional layer, and the polyamide-based film has a plasticity characteristic value represented by the above Equation 1 of 18.45 to 19.97%.

Advantageous Effects of the Invention

As the polyamide-based film according to the embodiments has a plasticity characteristic value within a predetermined range, its optical properties may be excellent, as well as its mechanical properties at the deep part and surface may be enhanced together. Accordingly, a polyamide-based film having excellent hardness, elasticity, and restoring force against indentation may be provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic exploded view of a display device according to an embodiment.

FIG. 2 is a schematic perspective view of a display device according to an embodiment.

FIG. 3 is a schematic cross-sectional view of a display device according to an embodiment.

FIG. 4 is a schematic flow diagram of a process for preparing a polyamide-based film according to an embodiment.

FIG. 5 is a schematic perspective view illustrating a tip applied to a nanoindenter for measuring the mechanical properties of a polyamide-based film.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings so that those skilled in the art to which the present invention pertains may easily practice them. However, the embodiments may be implemented in many different ways and are not limited to those described herein.

Throughout the present specification, in the case where each film, window, panel, layer, or the like is mentioned to be formed “on” or “under” another film, window, panel, layer, or the like, it means not only that one element is directly formed on or under another element, but also that one element is indirectly formed on or under another element with other element(s) interposed between them. In addition, the term on or under with respect to each element may be referenced to the drawings. For the sake of description, the sizes of individual elements in the appended drawings may be exaggeratedly depicted and do not indicate the actual sizes. In addition, the same reference numerals refer to the same elements throughout the specification.

Throughout the present specification, when a part is referred to as “comprising” an element, it is understood that other elements may be comprised, rather than other elements are excluded, unless specifically stated otherwise.

In the present specification, a singular expression is interpreted to cover a singular or plural number that is interpreted in context unless otherwise specified.

In addition, all numbers and expressions related to the quantities of components, reaction conditions, and the like used herein are to be understood as being modified by the term “about,” unless otherwise indicated.

The terms first, second, and the like are used herein to describe various elements, and the elements should not be limited by the terms. The terms are used only for the purpose of distinguishing one element from another.

In addition, the term “substituted” as used herein means to be substituted with at least one substituent group selected from the group consisting of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, an ester group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted alicyclic organic group, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted aryl group, and a substituted or unsubstituted heteroaryl group. The substituent groups enumerated above may be connected to each other to form a ring.

Polyamide-Based Film

The embodiments provide a polyamide-based film that is not only excellent in optical properties in terms of high transmittance, low haze, and low yellow index but also excellent in mechanical properties such as modulus, hardness, and restoring force.

The polyamide-based film according to an embodiment comprises a polyamide-base polymer.

The polyamide-based film has a plasticity characteristic value (or first plasticity characteristic value) represented by the following Equation 1 of 18.45 to 19.97%.

Plasticity characteristic value (%)=(100−nIT)/(HIT/HM)  [Equation 1]

In Equation 1, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm²), and HM is the composite hardness (N/mm²).

For example, in the force-indentation depth graph obtained by indenting a film with a nanoindenter and releasing the indentation, nIT (elasticity) may be defined as a percentage of the amount of work for the release of indentation to the sum of the amount of work for the indentation and the amount of work for the release of indentation. The amount of work for the release of indentation may be related to the amount of work by which the film deformed by the indentation is restored. Further, the amount obtained by subtracting the amount of work for the restoration from the amount of work for the total deformation may be defined as the amount of plastic work. The percentage of the amount of plastic work to the amount of work for the total deformation may be defined as plasticity.

The plasticity may be expressed by the following equation 4.

Plasticity (%)=(Amount of work for total deformation−amount of work for restoration)/(amount of work for total deformation)×100=100−nIT  [Equation 4]

HIT may indicate indentation hardness for a depth of tens to hundreds of nanometers from the surface of a film. For example, HIT may be related to a plastic characteristic of the elastic and plastic characteristics of a film.

HM is Martens hardness and may indicate composite characteristics of elasticity and plasticity against indentation.

Accordingly, the plasticity characteristic value of Equation 1 may be defined as a relationship between the plasticity, the plastic characteristic of the film surface, and the composite characteristics of elasticity and plasticity of the entire film. If the plasticity characteristic value is adjusted to 18.45 to 19.97%, optical properties such as light transmittance, haze, and yellow index and mechanical properties such as flexibility, resistance to indentation, and restorability to indentation of a polyamide-based film can be improved together.

In some embodiments, the plasticity characteristic value may be 18.45 to 19.95%, 18.45 to 19.93%, 18.5 to 19.97%, 18.5 to 19.95%, 18.5 to 19.93%, 18.7 to 19.97%, 18.7 to 19.95%, 18.7 to 19.93%, 19 to 19.97%, 19 to 19.95%, or 19 to 19.93%. In such a case, the optical properties and mechanical properties can be enhanced.

In some embodiments, the polyamide-based film may have an elasticity characteristic value represented by the following Equation 2 of 67 to 69.5%.

Elasticity characteristic value (%)=nIT/{EIT/(EIT+HIT)}  [Equation 2]

In Equation 2, nIT, EIT, and HIT are measured according to the ISO 14577 standard through a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).

EIT may indicate modulus for a depth of tens to hundreds of nanometers from the surface of a film. For example, EIT may be related to an elastic characteristic among the elastic and plastic characteristics of a film.

Accordingly, the elasticity characteristic value of Equation 2 may be defined as a relationship between the elastic and plastic components of the film surface and the restoring force. If the elasticity characteristic value is adjusted to 67 to 69.5%, optical properties such as light transmittance, haze, and yellow index and mechanical properties such as flexibility, resistance to indentation, and restorability to indentation of a polyamide-based film can be improved together.

Preferably, the elasticity characteristic value may be 67 to 69%, 67 to 68.8%, 67.5 to 69.5%, 67.5 to 69%, 67.5 to 68.8%, 68 to 69.5%, 68 to 69%, 68 to 68.8%, 68.2 to 69.5%, 68.2 to 69%, or 68.2 to 68.8%.

In some embodiments, the polyamide-based film may have a second plasticity characteristic value represented by the following Equation 3 of 384 to 416%.

Second plasticity characteristic value (%)=(1−nIT)/{HIT/(EIT+HIT)}  [Equation 3]

In Equation 3, nIT, EIT, and HIT are measured according to the ISO 14577 standard through a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).

The second plasticity characteristic value of Equation 3 may be defined as a relationship between the plasticity and the elastic and plastic components of the film surface. If the second plasticity characteristic value is adjusted to 384 to 416%, optical properties such as light transmittance, haze, and yellow index and mechanical properties such as flexibility, resistance to indentation, and restorability to indentation of a polyamide-based film can be improved together.

Preferably, the second plasticity characteristic value may be 384 to 412%, 384 to 410%, 388 to 416%, 388 to 412%, 388 to 410%, 390 to 416%, 390 to 412%, or 390 to 410%.

The polyamide-based film according to an embodiment may have an x-direction refractive index (n_(x)) of 1.60 to 1.70, 1.61 to 1.69, 1.62 to 1.68, 1.64 to 1.68, 1.64 to 1.66, or 1.64 to 1.65.

In addition, the polyamide-based film may have a y-direction refractive index (n_(y)) of 1.60 to 1.70, 1.61 to 1.69, 1.62 to 1.68, 1.63 to 1.68, 1.63 to 1.66, or 1.63 to 1.64.

Further, the polyamide-based film may have a z-direction refractive index (n_(z)) of 1.50 to 1.60, 1.51 to 1.59, 1.52 to 1.58, 1.53 to 1.58, 1.54 to 1.58, or 1.54 to 1.56.

If the x-direction refractive index, the y-direction refractive index, and the z-direction refractive index of the polyamide-based film are within the above ranges, when the film is applied to a display device, its visibility is excellent not only from the front but also from a lateral side, so that a wide angle of view can be achieved.

The polyamide-based film according to an embodiment may have an in-plane retardation (R_(o)) of 800 nm or less. Specifically, the in-plane retardation (R_(o)) of the polyamide-based film may be 700 nm or less, 600 nm or less, 550 nm or less, 100 nm to 800 nm, 200 nm to 800 nm, 200 nm to 700 nm, 300 nm to 700 nm, 300 nm to 600 nm, or 300 nm to 540 nm.

In addition, the polyamide-based film according to an embodiment may have a thickness direction retardation (R_(th)) of 5000 nm or less. Specifically, the thickness direction retardation (R_(th)) of the polyamide-based film may be 4,800 nm or less, 4,700 nm or less, 4,650 nm or less, 1,000 nm to 5,000 nm, 1,500 nm to 5,000 nm, 2,000 nm to 5,000 nm, 2,500 nm to 5,000 nm, 3,000 nm to 5,000 nm, 3,500 nm to 5,000 nm, 4,000 nm to 5,000 nm, 3,000 nm to 4,800 nm, 3,000 nm to 4,700 nm, 4,000 nm to 4,700 nm, or 4,200 nm to 4,650 nm.

Here, the in-plane retardation (R_(o)) is a parameter defined by a product (Δn_(xy)×d) of anisotropy (Δn_(xy)=|n_(x)−n_(y)|) of refractive indices of two mutually perpendicular axes on a film and the film thickness (d), which is a measure of the degree of optical isotropy and anisotropy.

In addition, the thickness direction retardation (R_(th)) is a parameter defined by a product of an average of the two birefringences Δn_(xz) (=|n_(x)−n_(z)|) and Δn_(yz) (=|n_(y)−n_(z)|) observed on a cross-section in the film thickness direction and the film thickness (d).

If the in-plane retardation and the thickness direction retardation of the polyamide-based film are within the above ranges, when the film is applied to a display device, it is possible to minimize the optical distortion and color distortion and to minimize the light leakage from a lateral side as well.

The polyamide-based film may comprise a filler in addition to the polyamide-base polymer.

The filler may have an average particle diameter of 60 nm to 180 nm. Specifically, the average particle diameter of the first filler may be 80 nm to 180 nm, 100 nm to 180 nm, 110 nm to 160 nm, 120 nm to 160 nm, or 130 nm to 150 nm, but it is not limited thereto.

If the average particle diameter of the filler is within the above range, the optical properties may not be deteriorated even when a relatively large amount thereof is employed as compared with other inorganic fillers.

The filler may have a refractive index of 1.55 to 1.75. Specifically, the refractive index of the filler may be 1.60 to 1.75, 1.60 to 1.70, 1.60 to 1.68, or 1.62 to 1.65, but it is not limited thereto.

If the refractive index of the filler satisfies the above range, the birefringence values related to n_(x), n_(y), and n_(z) can be appropriately adjusted, and the luminance of the film at various angles can be improved.

On the other hand, if the refractive index of the filler is outside the above range, there may arise a problem in that the filler is visually noticeable on the film or that the haze is increased due to the filler.

The content of the filler may be 100 ppm to 3,000 ppm based on the total weight of the solids content of the polyamide-based polymer. Specifically, the content of the filler may be 100 ppm to 2,500 ppm, 100 ppm to 2,200 ppm, 200 ppm to 2,500 ppm, 200 ppm to 2,200 ppm, 250 ppm to 2,100 ppm, or 300 ppm to 2,000 ppm, based on the total weight of the solids content of the polyamide-based polymer, but it is not limited thereto.

If the content of the filler is outside the above range, the haze of the film is steeply increased, and the filler may aggregate with each other on the surface of the film, so that a feeling of foreign matter may be visually observed, or it may cause a trouble in the sliding performance or deteriorate the windability in the preparation process.

The filler may comprise, for example, silica and barium sulfate, but it is not limited thereto.

The filler may be employed in the form of particles. In addition, the surface of the filler is not subjected to special coating treatment, and it may be uniformly dispersed in the entire film.

As the polyamide-based film comprises the filler, the film can secure a wide angle of view without a deterioration in the optical properties.

The content of residual solvents in the polyamide-based film may be 1,500 ppm or less. For example, the content of residual solvents may be 1,200 ppm or less, 1,000 ppm or less, 800 ppm or less, or 500 ppm or less, but it is not limited thereto.

The residual solvent refers to a solvent that has not been volatilized during the film production and remains in the film finally produced.

If the content of the residual solvents in the polyamide-based film exceeds the above range, the durability of the film may be deteriorated, and it may have an impact on the luminance.

When the polyamide-based film according to an embodiment based on a thickness of 50 μm is folded to have a radius of curvature of 3 mm, the number of folding before the fracture may be 200,000 or more.

The number of folding counts one when the film is folded to have a radius of curvature of 3 mm and then unfolded.

As the number of folding of the polyamide-based film satisfies the above range, it can be advantageously applied to a foldable display device or a flexible display device.

The polyamide-based film according to an embodiment may have a surface roughness of 0.01 μm to 0.07 μm. Specifically, the surface roughness may be 0.01 μm to 0.07 μm or 0.01 μm to 0.06 μm, but it is not limited thereto.

As the surface roughness of the polyamide-based film satisfies the above range, it may be advantageous for achieving high luminance even when the angle from the normal direction of a surface light source is increased.

The polyamide-based film according to an embodiment comprises a polyamide-based polymer, and the polyamide-based polymer may comprise an amide repeat unit and, optionally, may comprise an imide repeat unit.

In some embodiments, the polyamide-based polymer may comprise an amide-based repeat unit and an imide-based repeat unit at a molar ratio of 100:0 to 90:0. In such a case, mechanical properties such as flexibility, mechanical strength, and resistance/restoring force to indentation and optical properties such as transparency, transmittance, and haze of the polyamide-based film may be improved together. Preferably, the polyamide-based polymer may comprise an amide-based repeat unit and an imide-based repeat unit at a molar ratio of 99:1 to 97:3 or 99:1 to 95:5.

The polyamide-based polymer may be prepared by simultaneously or sequentially reacting reactants that comprise a diamine compound and a dicarbonyl compound. Specifically, the polyamide-based polymer may be prepared by polymerizing a diamine compound and a dicarbonyl compound.

Alternatively, the polyamide-based polymer is prepared by polymerizing a diamine compound, a dianhydride compound, and a dicarbonyl compound. Here, the polyamide-based polymer may comprise an imide repeat unit derived from the polymerization of the diamine compound and the dianhydride compound and an amide repeat unit derived from the polymerization of the diamine compound and the dicarbonyl compound.

The polyamide-based film according to an embodiment may comprise a polyamide-based polymer in which an amide bond is formed by the polymerization of a diamine compound and a dicarbonyl compound. The polyamide-based film may optionally comprise a polyamide-imide-based polymer in which an imide bond is formed by the further polymerization of a dianhydride compound.

The diamine compound is a compound that forms an imide bond with the dianhydride compound and forms an amide bond with the dicarbonyl compound, to thereby form a copolymer.

The diamine compound is not particularly limited, but it may be, for example, an aromatic diamine compound that contains an aromatic structure. For example, the diamine compound may be a compound represented by the following Formula 1.

H₂N-(E)_(e)-NH₂  [Formula 1]

In Formula 1,

E may be selected from a substituted or unsubstituted divalent C₆-C₃₀ aliphatic cyclic group, a substituted or unsubstituted divalent C₄-C₃₀ heteroaliphatic cyclic group, a substituted or unsubstituted divalent C₆-C₃₀ aromatic cyclic group, a substituted or unsubstituted divalent C₄-C₃₀ heteroaromatic cyclic group, a substituted or unsubstituted C₁-C₃₀ alkylene group, a substituted or unsubstituted C₂-C₃₀ alkenylene group, a substituted or unsubstituted C₂-C₃₀ alkynylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O)₂—, —Si(CH₃)₂—, —C(CH₃)₂—, and —C(CF₃)₂—.

e is selected from integers of 1 to 5. When e is 2 or more, the Es may be the same as, or different from, each other.

(E)_(e) in Formula 1 may be selected from the groups represented by the following Formulae 1-1a to 1-14a, but it is not limited thereto.

Specifically, (E)_(e) in Formula 1 may be selected from the groups represented by the following Formulae 1-1b to 1-13b, but it is not limited thereto.

More specifically, (E)_(e) in Formula 1 may be the group represented by the above Formula 1-6b or the group represented by the above Formula 1-9b.

In an embodiment, the diamine compound may comprise a compound having a fluorine-containing substituent or a compound having an ether group (—O—).

The diamine compound may be composed of a compound having a fluorine-containing substituent. In such event, the fluorine-containing substituent may be a fluorinated hydrocarbon group and specifically may be a trifluoromethyl group. But it is not limited thereto.

In another embodiment, one kind of diamine compound may be used as the diamine compound. That is, the diamine compound may be composed of a single component.

For example, the diamine compound may comprise 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) represented by the following formula, but it is not limited thereto.

The dicarbonyl compound is not particularly limited, but it may be, for example, a compound represented by the following Formula 3.

In Formula 3,

J may be selected from a substituted or unsubstituted divalent C₆-C₃₀ aliphatic cyclic group, a substituted or unsubstituted divalent C₄-C₃₀ heteroaliphatic cyclic group, a substituted or unsubstituted divalent C₆-C₃₀ aromatic cyclic group, a substituted or unsubstituted divalent C₄-C₃₀ heteroaromatic cyclic group, a substituted or unsubstituted C₁-C₃₀ alkylene group, a substituted or unsubstituted C₂-C₃₀ alkenylene group, a substituted or unsubstituted C₂-C₃₀ alkynylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O)₂—, —Si(CH₃)₂—, —C(CH₃)₂—, and —C(CF₃)₂—.

j is selected from integers of 1 to 5. When j is 2 or more, the Js may be the same as, or different from, each other.

X is a halogen atom. Specifically, X may be F, Cl, Br, I, or the like. More specifically, X may be Cl, but it is not limited thereto.

(J)_(j) in the above Formula 3 may be selected from the groups represented by the following Formulae 3-1a to 3-14a, but it is not limited thereto.

Specifically, (J)_(j) in the above Formula 3 may be selected from the groups represented by the following Formulae 3-1b to 3-8b, but it is not limited thereto.

More specifically, (J)_(j) in Formula 3 may be the group represented by the above Formula 3-1b, the group represented by the above Formula 3-2b, the group represented by the above Formula 3-3b, or the group represented by the above Formula 3-8b.

In an embodiment, a mixture of at least two kinds of dicarbonyl compounds different from each other may be used as the dicarbonyl compound. If two or more dicarbonyl compounds are used, at least two dicarbonyl compounds in which (J)_(j) in the above Formula 3 is selected from the groups represented by the above Formulae 3-1b to 3-8b may be used as the dicarbonyl compound.

In another embodiment, the dicarbonyl compound may be an aromatic dicarbonyl compound that contains an aromatic structure.

The dicarbonyl compound may comprise terephthaloyl chloride (TPC), 1,1′-biphenyl-4,4′-dicarbonyl dichloride (BPDC), isophthaloyl chloride (IPC), as represented by the following formulae, or a combination thereof. But it is not limited thereto. Preferably, TPC and IPC may be used together.

In an embodiment, the polyamide-based polymer may comprise two or more types of an amide-based repeat unit.

For example, the two or more types of an amide-based repeat unit may comprise a first amide-based repeat unit and a second amide-based repeat unit. The first amide-based repeat unit may be formed by reacting a first dicarbonyl compound with the diamine compound. The second amide-based repeat unit may be formed by reacting a second dicarbonyl compound with the diamine compound.

The first dicarbonyl compound and the second dicarbonyl compound may be compounds different from each other.

The first dicarbonyl compound and the second dicarbonyl compound may comprise two carbonyl groups, respectively. The angle between the two carbonyl groups contained in the first dicarbonyl compound may be greater than the angle between the two carbonyl groups contained in the second dicarbonyl compound.

In some examples, the first dicarbonyl compound and the second dicarbonyl compound may be structural isomers to each other. As two kinds of dicarbonyl compounds in a structural isomeric relationship are used, a polyamide-based polymer and a film satisfying Equation 1, Equation 2, and/or Equation 3 above can be formed, thereby enhancing the optical properties and mechanical properties of the polyamide-based polymer.

The first dicarbonyl compound and the second dicarbonyl compound may be an aromatic dicarbonyl compound, respectively. In some embodiments, the first dicarbonyl compound and the second dicarbonyl compound may each have one benzene ring (a phenyl group).

For example, the first dicarbonyl compound and the second dicarbonyl compound may be aromatic dicarbonyl compounds different from each other, but they are not limited thereto.

If the first dicarbonyl compound and the second dicarbonyl compound are an aromatic dicarbonyl compound, respectively, they comprise a benzene ring. Thus, they can contribute to improvements in the mechanical properties such as surface hardness and tensile strength of a film that comprises the polyamide-based polymer thus produced.

For example, the angle between the two carbonyl groups contained in the first dicarbonyl compound may be 160 to 180°, and the angle between the two carbonyl groups contained in the second dicarbonyl compound may be 80 to 140°.

For example, the dicarbonyl compound may comprise a first dicarbonyl compound and/or a second dicarbonyl compound.

For example, the first dicarbonyl compound may comprise TPC, and the second dicarbonyl compound may comprise IPC, but they are not limited thereto.

If TPC is used as the first dicarbonyl compound and IPC is used as the second dicarbonyl compound in a proper combination, a film that comprises the polyamide-based resin thus produced may have high oxidation resistance, productivity, light transmittance, transparency, hardness, and modulus, low haze, and excellent resistance and restoring force to indentation.

The diamine compound and the dicarbonyl compound may be polymerized to form a repeat unit represented by the following Formula B.

In Formula B, E, J, e, and j are as described above.

For example, the diamine compound and the dicarbonyl compound may be polymerized to form amide repeat units represented by the following Formulae B-1 and B-2.

In Formula B-1, y is an integer of 1 to 400.

In Formula B-2, y is an integer of 1 to 400.

In some embodiments, the molar ratio of the first amide-based repeat unit to the second amide-based repeat unit may be 25:75 to 75:25. As the molar ratio of the first and second amide-based repeat units is set to the above range, the plasticity characteristic value and elasticity characteristic value of the polyamide-based polymer may be adjusted to ranges satisfying Equations 1 to 3 above. Accordingly, it is possible to improve the haze, light transmittance, yellow index, modulus, hardness, and restoring force of the polyamide-based film. Preferably, the molar ratio of the first amide-based repeat unit to the second amide-based repeat unit may be 30:70 to 75:25, 35:65 to 75:25, or 40:60 to 75:25.

The dianhydride compound is not particularly limited, but it may comprise a cyclic dianhydride compound. For example, the dianhydride compound may reduce the birefringence characteristics of the polyamide-based resin and enhance such optical properties as transmittance of the polyamide-based film.

In some embodiments, the cyclic dianhydride compound may comprise an alicyclic dianhydride compound or an aromatic dianhydride compound.

The alicyclic dianhydride compound may comprise a compound in which two anhydride groups are substituted in the alicyclic ring structure. The alicyclic ring structure may contain 4 to 12 carbons and may be fully saturated or partially unsaturated. Specifically, the alicyclic dianhydride compound may comprise, for example, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA).

The aromatic dianhydride compound may comprise a compound in which two anhydride groups are substituted in an aromatic ring having 6 to 30 carbon atoms. The aromatic ring may comprise benzene, a fused ring such as naphthalene, or a linked ring such as biphenyl. For example, the aromatic dianhydride compound may comprise, for example, 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA).

The two anhydride groups in the dianhydride compound may be directly substituted to the cyclic hydrocarbon group or may be substituted at positions symmetrical to each other in the alicyclic ring or the aromatic ring. In such a case, the hardness and restoring force to indentation of the polyamide-based film may be enhanced.

In an embodiment, the dianhydride compound may not comprise a fluorine-containing substituent. For example, when the dianhydride compound contains a fluorine group, the plastic component of the film may be excessively increased; thus, its flexibility and restoring force to indentation may be reduced.

For example, the dianhydride compound may comprise a compound represented by the following Formula 2.

In Formula 2, G is a substituted or unsubstituted tetravalent C₆-C₁₂ aliphatic cyclic group, a substituted or unsubstituted tetravalent C₄-C₁₂ heteroaliphatic cyclic group, a substituted or unsubstituted tetravalent C₆-C₃₀ aromatic cyclic group, or a substituted or unsubstituted tetravalent C₄-C₃₀ heteroaromatic cyclic group, wherein the aliphatic cyclic group, the heteroaliphatic cyclic group, the aromatic cyclic group, or the heteroaromatic cyclic group may be present alone, fused to each other to form a condensed ring, or bonded by a bonding group selected from —O—, —S—, —C(═O)—, and —S(═O)₂—.

G in the above Formula 2 may be selected from the groups represented by the following Formulae 2-1a to 2-9a, but it is not limited thereto.

For example, G in Formula 2 may be the group represented by the above Formula 2-1a or the group represented by the above Formula 2-5a.

The diamine compound and the dianhydride compound may be polymerized to form an amic acid group.

Subsequently, the amic acid group may be converted to an imide group through a dehydration reaction. In such a case, a polyamide-imide-based polymer comprising a polyimide segment and a polyamide segment may be formed.

In some embodiments, the polyamide-based film has a thickness deviation of 4 μm or less based on a thickness of 50 μm. The thickness deviation may refer to a deviation between the maximum or minimum value with respect to the average of thicknesses measured at 10 random positions of the film. In such a case, as the polyamide-based film has a uniform thickness, its optical properties and mechanical properties at each point may be uniformly exhibited.

The polyamide-based film may have a haze of 1% or less. For example, the haze may be 0.7% or less or 0.5% or less, but it is not limited thereto.

The polyamide-based film may have a transmittance of 80% or more. For example, the transmittance may be 82% or more, 85% or more, 88% or more, 89% or more, 80% to 99%, 88% to 99%, or 89% to 99%, but it is not limited thereto.

The polyamide-based film may have a yellow index of 3 or less. For example, the yellow index may be 2.8 or less or 2.5 or less, but it is not limited thereto.

The polyamide-based film may have a modulus of 6 GPa or more. Specifically, the modulus may be 6.3 GPa or more or 6.5 GPa or more, but it is not limited thereto.

The polyamide-based film may have a compressive strength of 0.4 kgf/μm or more. Specifically, the compressive strength may be 0.45 kgf/μm or more, or 0.46 kgf/μm or more, but it is not limited thereto.

When the polyamide-based film is perforated at a speed of 10 mm/min using a 2.5-mm spherical tip in a UTM compression mode, the maximum diameter (mm) of perforation including a crack is 60 mm or less. Specifically, the maximum diameter of perforation may be 5 to 60 mm, 10 to 60 mm, 15 to 60 mm, 20 to 60 mm, 25 to 60 mm, or 25 to 58 mm, but it is not limited thereto.

The polyamide-based film may have a surface hardness of HB or higher. Specifically, the surface hardness may be H or higher, or 2H or higher, but it is not limited thereto.

The polyamide-based film may have a tensile strength of 15 kgf/mm² or more. Specifically, the tensile strength may be 18 kgf/mm² or more, 20 kgf/mm² or more, 21 kgf/mm² or more, or 22 kgf/mm² or more, but it is not limited thereto.

The polyamide-based film may have an elongation of 15% or more. Specifically, the elongation may be 16% or more, 17% or more, or 17.5% or more, but it is not limited thereto.

The polyamide-based film according to an embodiment has excellent optical properties in terms of low haze, low yellow index, and high transmittance, as well as excellent mechanical properties such as modulus, hardness, and elasticity.

The physical properties of the polyamide-based film as described above are based on a thickness of 40 μm to 60 μm. For example, the physical properties of the polyamide-based film are based on a thickness of 50 μm.

The features on the components and properties of the polyamide-based film as described above may be combined with each other.

For example, the polyamide-based film comprises a polyamide-based polymer and may have a transmittance of 80% or more, a haze of 1% or less, and a yellow index of 3 or less.

In addition, the plasticity characteristic value and elasticity characteristic value of Equations 1 to 3 of the polyamide-based film as described above may be adjusted by combinations of the chemical and physical properties of the components, which constitute the polyamide-based film, along with the conditions in each step of the process for preparing the polyamide-based film as described below.

For example, the composition and content of the components that constitute the polyamide-based film, the polymerization conditions and thermal treatment conditions in the film preparation process, and the like are all combined to achieve the plasticity characteristic value and/or elasticity characteristic value in desired ranges.

Cover Window for a Display Device

The cover window for a display device according to an embodiment comprises a polyamide-based film and a functional layer.

The polyamide-based film comprises a polyamide-based polymer, wherein the plasticity characteristic value represented by the above Equation 1 is 18.45 to 19.97%.

Details on the polyamide-based film are as described above.

The cover window for a display device can be advantageously applied to a display device.

As the polyamide-based film has a plasticity characteristic value and/or an elasticity characteristic value within the above ranges, it may have excellent optical and mechanical properties.

Display Device

The display device according to an embodiment comprises a display unit; and a cover window disposed on the display unit, wherein the cover window comprises a polyamide-based film and a functional layer.

The polyamide-based film comprises a polyamide-based polymer, wherein the plasticity characteristic value represented by the above Equation 1 is 18.45 to 19.97%.

Details on the polyamide-based film and the cover window are as described above.

FIG. 1 is a schematic exploded view of a display device according to an embodiment. FIG. 2 is a schematic perspective view of a display device according to an embodiment. FIG. 3 is a schematic cross-sectional view of a display device according to an embodiment.

Specifically, FIGS. 1 to 3 illustrate a display device, which comprises a display unit (400) and a cover window (300) disposed on the display unit (400), wherein the cover window comprises a polyamide-based film (100) having a first side (101) and a second side (102) and a functional layer (200), and an adhesive layer (500) is interposed between the display unit (400) and the cover window (300).

The display unit (400) is for displaying an image, and it may have flexible characteristics.

The display unit (400) may be a display panel for displaying an image. For example, it may be a liquid crystal display panel or an organic electroluminescent display panel. The organic electroluminescent display panel may comprise a front polarizing plate and an organic EL panel.

The front polarizing plate may be disposed on the front side of the organic EL panel. Specifically, the front polarizing plate may be attached to the side on which an image is displayed in the organic EL panel.

The organic EL panel may display an image by self-emission of a pixel unit. The organic EL panel may comprise an organic EL substrate and a driving substrate. The organic EL substrate may comprise a plurality of organic electroluminescent units, each of which corresponds to a pixel. Specifically, it may comprise a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, and an anode. The driving substrate is operatively coupled to the organic EL substrate. That is, the driving substrate may be coupled to the organic EL substrate so as to apply a driving signal such as a driving current, so that the driving substrate can drive the organic EL substrate by applying a current to the respective organic electroluminescent units.

In addition, an adhesive layer (500) may be interposed between the display unit (400) and the cover window (300). The adhesive layer may be an optically transparent adhesive layer, but it is not particularly limited.

The cover window (300) may be disposed on the display unit (400). The cover window is located at the outermost position of the display device to thereby protect the display unit.

The cover window (300) may comprise a polyamide-based film and a functional layer. The functional layer may be at least one selected from the group consisting of a hard coating layer, a reflectance reducing layer, an antifouling layer, and an antiglare layer. The functional layer may be coated on at least one side of the polyamide-based film.

The polyamide-based film according to an embodiment can be applied in the form of a film to the outside of a display device without changing the display driving method, the color filter inside the panel, or the laminated structure, thereby providing a display device having a uniform thickness, low haze, high transmittance, and high transparency. Since neither significant process changes nor cost increases are needed, it is advantageous in that the production costs can be reduced.

The polyamide-based film according to an embodiment may be excellent in optical properties in terms of high transmittance, low haze, and low yellow index, as well as may have excellent mechanical properties such as hardness, modulus, and elasticity.

In addition, the polyamide-based film according to an embodiment can minimize the optical distortion since it has at most a certain level of in-plane retardation and a thickness direction retardation and can also reduce the light leakage from a lateral side.

A polyamide-based film having a plasticity characteristic value and/or an elastic characteristic value within the above ranges may have excellent resistance and restoring force to indentation as well as high flexibility. Thus, when applied to a cover window of a touch-type display device to which pressure is continuously applied by a hand or a stylus pen, it may have excellent durability and lifespan.

Process for Preparing a Polyamide-Based Film

An embodiment provides a process for preparing a polyamide-based film.

The process for preparing a polyamide-based film according to an embodiment comprises polymerizing a diamine compound and a dicarbonyl compound to prepare a solution comprising a polyamide-based polymer in an organic solvent; casting the polymer solution and then drying it to prepare a gel sheet; and thermally treating the gel sheet.

Referring to FIG. 4, the process for preparing a polyamide-based film according to an embodiment comprises polymerizing a diamine compound and a dicarbonyl compound in an organic solvent to prepare a solution comprising a polyamide-based polymer (S100); casting the polymer solution to prepare a gel sheet (S200); and thermally treating the gel sheet (S300).

The process for preparing a polyamide-based film according to some embodiments may further comprise adjusting the viscosity of the polyamide-based polymer solution (S110), aging the polyamide-based polymer solution (S120), and/or degassing the polyamide-based polymer solution (S130).

The polyamide-based film is a film in which a polyamide-based polymer is a main component. The polyamide-based polymer is a resin that comprises an amide repeat unit as a structural unit. Optionally, the polyamide-based polymer may comprise an imide repeat unit.

In the process for preparing a polyamide-based film, the polymer solution for preparing a polyamide-based polymer may be prepared by simultaneously or sequentially mixing a diamine compound and a dicarbonyl compound in an organic solvent in a reactor, and reacting the mixture (S100).

In an embodiment, the step of preparing the polymer solution may comprise mixing and reacting the diamine compound and the dicarbonyl compound in a solvent to produce a polyamide (PA) solution. The polyamide solution is a solution that comprises a polymer having an amide repeat unit.

In an embodiment, the step of preparing the polymer solution may be carried out by using two kinds of dicarbonyl compounds different from each other as the dicarbonyl compound. In such a case, the two kinds of dicarbonyl compounds may be mixed and reacted simultaneously or sequentially. Preferably, the first dicarbonyl compound and the diamine compound may react to form a prepolymer, and the prepolymer and the second dicarbonyl compound may react to form the polyamide-based polymer. In such a case, the plasticity characteristic value and/or elasticity characteristic value of the polyamide-based polymer may be readily adjusted.

The polymer contained in the polymer solution comprises an amide repeat unit derived from the polymerization of the diamine compound and the dicarbonyl compound.

Details on the diamine compound and the dicarbonyl compound are as described above.

In an embodiment, the process may further comprise adjusting the viscosity of the polymer solution (S110) after the step of preparing the polymer solution. The viscosity of the polymer solution may be 200,000 cps to 350,000 cps at room temperature. In such an event, the film-forming capability of a polyamide-based film can be enhanced, thereby enhancing the thickness uniformity.

Specifically, the step of preparing the polymer solution may comprise simultaneously or sequentially mixing and reacting a diamine compound and a dicarbonyl compound in an organic solvent to prepare a first polymer solution; and further adding the dicarbonyl compound to prepare a second polymer solution having the target viscosity.

In the steps of preparing the first polymer solution and the second polymer solution, the polymer solutions may have viscosities different from each other. For example, the second polymer solution may have a viscosity higher than that of the first polymer solution.

In the steps of preparing the first polymer solution and the second polymer solution, the stirring speeds may be different from each other. For example, the stirring speed when the first polymer solution is prepared may be faster than the stirring speed when the second polymer solution is prepared.

In some embodiment, when the first polymer solution is prepared, the first dicarbonyl compound and the second dicarbonyl compound may be sequentially reacted with the diamine compound. In such an event, a first polymer solution having a viscosity of 10,000 to 150,000 cps may be formed. Preferably, the viscosity of the first polymer solution may be 10,000 to 100,000 cps, 10,000 to 80,000 cps, 10,000 to 60,000 cps, 20,000 to 150,000 cps, 20,000 to 100,000 cps, 20,000 to 80,000 cps, 20,000 to 60,000 cps, 40,000 to 150,000 cps, 40,000 to 100,000 cps, 40,000 to 80,000 cps, or 40,000 to 60,000 cps.

In some embodiments, the second dicarbonyl compound may be further reacted with the first polymer solution to form a second polymer solution having a viscosity of 200,000 to 350,000 cps. For example, the step of further reacting the second dicarbonyl compound may serve as the step of viscosity adjustment.

As the viscosity of the polyamide-based polymer solution is adjusted through the first polymer solution and the second polymer solution, the plasticity characteristic value and elasticity characteristic value in desired ranges can be effectively achieved.

For example, the viscosity of the polymer solution, plasticity characteristic value, and elasticity characteristic value may be adjusted by controlling the sequence and amounts of the first dicarbonyl compound and the second dicarbonyl compound added.

The molar ratio of the first dicarbonyl compound to the second dicarbonyl compound for the preparation of the polymer solution may be 25:75 to 75:25, preferably, 30:70 to 75:25, 35:65 to 75:25, or 40:60 to 75:25.

As the first dicarbonyl compound and the second dicarbonyl compound are used at such a ratio, it is possible to prepare a polyamide-based polymer having a plasticity characteristic value and an elasticity characteristic value within the above ranges and to improve the modulus, haze, transmittance, yellow index, and the like of the polyamide-based film.

In some embodiments, when the polymer solution (first polymer solution) is formed, a dianhydride compound is reacted with a diamine compound to form a polyamic acid or a polyimide. Then, the polyamic acid or polyimide may be reacted with a dicarbonyl compound to form a polymer solution comprising a polyamide-imide. In such an event, the dianhydride compound as described above may be used. Its amount used may be 1 to 10% by mole, preferably, 1 to 3% by mole or 1 to 5% by mole, based on the total amount of the dicarbonyl compound and the dianhydride compound. In such a case, the viscosities of the polymer solution, the first polymer solution, and the second polymer solution can be adjusted to desired ranges, and a polyamide-based film having a plasticity characteristic value and/or an elasticity characteristic value in the desired ranges can be prepared.

In an embodiment, the mixing and reaction of the solvent, the diamine compound, and the dicarbonyl compound may be carried out at a temperature of −20 to 25° C. If it is outside the above temperature range, excessively few or many polymerization nuclei are formed, thereby making it difficult to form a polyamide-based polymer having the desired plastic characteristics and elastic characteristics. Thus, the mechanical properties and optical properties of the polyamide-based film may be deteriorated. In addition, the viscosity of the polymer solution may be less than a predetermined range. Preferably, the mixing and reaction of the diamine compound and the dicarbonyl compound may be carried out at a temperature of −20 to 20° C., −20 to 15° C., −20 to 10° C., −15 to 20° C., −15 to 15° C., −15 to 10° C., −10 to 20° C., −10 to 15° C., −10 to 10° C., −8 to 20° C., −8 to 15° C., −8 to 10° C., −5 to 20° C., −5 to 15° C., or −5 to 10° C.

In an embodiment, the mixing and reaction of the diamine compound and the dianhydride compound may be carried out at a temperature of 0 to 50° C. If it is outside the above temperature range, excessively few or many polymerization nuclei are formed, thereby making it difficult to form a polyamide-based polymer having the desired plastic characteristics and elastic characteristics. Thus, the mechanical properties and optical properties of the polyamide-based film may be deteriorated. In addition, the viscosity of the polymer solution may be less than a predetermined range. Preferably, the mixing and reaction of the diamine compound and the dianhydride compound may be carried out at a temperature of 0 to 45° C., 0 to 40° C., 10 to 50° C., 10 to 45° C., 10 to 40° C., 20 to 50° C., 20 to 45° C., or 20 to 40° C.

The content of solids contained in the polymer solution may be 10% by weight to 30% by weight, but it is not limited thereto.

If the content of solids contained in the polymer solution is within the above range, a polyamide-based film can be effectively produced in the casting step. In addition, the polyamide-based film thus produced may have mechanical properties in terms of an improved modulus and the like and optical properties in terms of a low yellow index and the like.

In still another embodiment, the step of preparing the polymer solution may further comprise adjusting the pH of the polymer solution. In this step, the pH of the polymer solution may be adjusted to 4 to 7, for example, 4.5 to 7.

The pH of the polymer solution may be adjusted by adding a pH adjusting agent. The pH adjusting agent is not particularly limited and may include, for example, amine-based compounds such as alkoxyamine, alkylamine, and alkanolamine.

As the pH of the polymer solution is adjusted to the above range, it is possible to prevent the occurrence of defects in the film produced from the polymer solution and to achieve the desired optical properties and mechanical properties in terms of yellow index and modulus.

The pH adjusting agent may be employed in an amount of 0.1% by mole to 10% by mole based on the total number of moles of monomers in the polymer solution.

In an embodiment, the organic solvent may be at least one selected from the group consisting of dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), m-cresol, tetrahydrofuran (THF), and chloroform. The organic solvent employed in the polymer solution may be dimethylacetamide (DMAc), but it is not limited thereto.

The polymer solution may be stored at −20° C. to 20° C., −20° C. to 10° C., −20° C. to 5° C., −20° C. to 0° C., or 0° C. to 10° C.

If it is stored at the above temperature, it is possible to prevent degradation of the polymer solution and to lower the moisture content to thereby prevent defects of a film produced therefrom.

In some embodiments, the polymer solution or the polymer solution whose viscosity has been adjusted may be aged (S120).

The aging may be carried out by leaving the polymer solution at a temperature of −10 to 10° C. for 24 hours or longer. In such an event, the polyamide-based polymer or unreacted materials contained in the polymer solution, for example, may complete the reaction or achieve a chemical equilibrium, whereby the polymer solution may be homogenized. The mechanical properties, optical properties, and surface energy characteristics of a polyamide-based film formed therefrom may be substantially uniform over the entire area of the film. Preferably, the aging may be carried out at a temperature of −5 to 10° C., −5 to 5° C., or −3 to 5° C.

In an embodiment, the process may further comprise degassing the polyamide-based polymer solution (S130). The step of degassing may remove moisture in the polymer solution and reduce impurities, thereby increasing the reaction yield and imparting excellent surface appearance and mechanical properties to the film finally produced.

The degassing may comprise vacuum degassing or purging with an inert gas.

The vacuum degassing may be carried out for 30 minutes to 3 hours after depressurizing the internal pressure of the tank in which the polymer solution is contained to 0.1 bar to 0.7 bar. The vacuum degassing under these conditions may reduce bubbles in the polymer solution. As a result, it is possible to prevent surface defects of the film produced therefrom and to achieve excellent optical properties such as haze.

Specifically, the purging may be carried out by purging the tank with an inert gas at an internal pressure of 1 atm to 2 atm. The purging under these conditions may remove moisture in the polymer solution, reduce impurities to thereby increase the reaction yield, and achieve excellent optical properties such as haze and mechanical properties.

The inert gas may be at least one selected from the group consisting of nitrogen, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), but it is not limited thereto. Specifically, the inert gas may be nitrogen.

The vacuum degassing and the purging with an inert gas may be carried out in separate steps.

For example, the step of vacuum degassing may be carried out, followed by the step of purging with an inert gas, but it is not limited thereto.

The vacuum degassing and/or the purging with an inert gas may improve the physical properties of the surface of a polyamide-based film thus produced.

Once the solution comprising a polyamide-based polymer in an organic solvent has been prepared as described above, a filler may be added to the solution.

The filler has an average particle diameter of 60 nm to 180 nm and a refractive index of 1.55 to 1.75. The content thereof is 100 ppm to 3,000 ppm based on the total weight of the solids content of the polyamide-based polymer. In addition, the filler may be silica or barium sulfate.

Details on the filler are as described above.

The polymer solution may be cast to prepare a gel sheet (S200).

For example, the polymer solution may be extruded, coated, and/or dried on a support to form a gel sheet.

In addition, the casting thickness of the polymer solution may be 200 μm to 700 μm. As the polymer solution is cast to a thickness within the above range, the final film produced after the drying and thermal treatment may have an appropriate and uniform thickness.

The polymer solution may have a viscosity of 200,000 cps to 350,000 cps at room temperature as described above. As the viscosity satisfies the above range, the polymer solution can be cast to a uniform thickness without defects, and a polyamide-based film having a substantially uniform thickness can be formed without local/partial thickness variations during drying. In addition, a film having a plastic characteristic and an elastic characteristic as described above can be produced.

The polymer solution is cast and then dried at a temperature of 60° C. to 150° C. for 5 minutes to 60 minutes to prepare a gel sheet. Specifically, the polymer solution is dried at a temperature of 70° C. to 90° C. for 15 minutes to 40 minutes to prepare a gel sheet.

The solvent of the polymer solution may be partially or totally volatilized during the drying to prepare the gel sheet.

The dried gel sheet may be thermally treated to form a polyamide-based film (S300).

The thermal treatment of the gel sheet may be carried out, for example, through a thermosetting device.

The thermosetting device may thermally treat the gel sheet through hot air.

If the thermal treatment is carried out with hot air, the heat may be uniformly supplied. If the heat is not uniformly supplied, a satisfactory surface roughness cannot be achieved, which may raise or lower the surface energy too much.

The thermal treatment of the gel sheet may be carried out in a temperature range of 60° C. to 500° C. for 5 minutes to 200 minutes. Specifically, the thermal treatment of the gel sheet may be carried out in a temperature range of 80° C. to 300° C. at a temperature elevation rate of 1.5° C./minute to 20° C./minute for 10 minutes to 150 minutes.

In such an event, the initial temperature of the thermal treatment of the gel sheet may be 60° C. or higher. Specifically, the initial temperature of the thermal treatment of the gel sheet may be 80° C. to 180° C.

In addition, the maximum temperature in the thermal treatment may be 300° C. to 500° C. For example, the maximum temperature in the thermal treatment may be 350° C. to 500° C., 380° C. to 500° C., 400° C. to 500° C., 410° C. to 480° C., 410° C. to 470° C., or 410° C. to 450° C.

According to an embodiment, the thermal treatment of the gel sheet may be carried out in two or more steps.

Specifically, the thermal treatment may comprise a first hot air treatment step carried out for 5 minutes to 30 minutes in a range of 60° C. to 120° C.; and a second hot air treatment step carried out for 10 minutes to 120 minutes in a range of 120° C. to 350° C.

The thermal treatment under these conditions may cure the gel sheet to have appropriate surface hardness, modulus, and surface energy and may secure high light transmittance, low haze, and an appropriate level of glossiness of the cured film at the same time.

According to an embodiment, the thermal treatment may comprise passing it through an IR heater. The thermal treatment by an IR heater may be carried out for 1 minute to 30 minutes in a temperature range of 300° C. or higher. Specifically, the thermal treatment by an IR heater may be carried out for 1 minute to 20 minutes in a temperature range of 300° C. to 500° C.

The polyamide-based film is prepared by the preparation process as described above such that it may satisfy Equations 1, 2, and/or 3 as described above and may be excellent in optical properties and mechanical properties.

The polyamide-based film can be applied to various uses that require flexibility, transparency, and a certain level of luminance. For example, the polyamide-based film can be applied to solar cells, displays, semiconductor devices, sensors, and the like.

In particular, since the polyamide-based film has excellent thickness uniformity and optical properties, it can be advantageously applied to a cover window for a display device and to a display device. Since it has excellent folding characteristics, it can be advantageously applied to a foldable display device or a flexible display device.

Details on the polyamide-based film prepared by the process for preparing a polyamide-based film are as described above.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, these examples are set forth to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example and Comparative Example

A 1-liter glass reactor equipped with a temperature-controllable double jacket was charged with 675 g of dimethylacetamide (DMAc) as an organic solvent at 10° C. under a nitrogen atmosphere. Then, 64.0 g (0.200 mole) of 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB) as an aromatic diamine was slowly added thereto and dissolved.

Subsequently, the temperature inside the reactor was raised to 30° C., and 1.77 g (0.006 mole) of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) was slowly added while the reaction solution was stirred for 2 hours.

The temperature of the reactor was lowered to 10° C., and 30.37 g (0.15 mole) of terephthaloyl chloride (TPC) was slowly added while the mixture was stirred for 1 hour. Then, 9.13 g (0.05 mole; 94% by mole of the total amount introduced) of isophthaloyl chloride (IPC) was added, followed by stirring the mixture for 1 hour, thereby preparing a first polymer solution. The viscosity of the first polymer solution thus prepared was about 50,000 cps.

Then, 1 ml of an IPC solution having a concentration of 10% by weight in a DMAc solvent was added thereto, followed by stirring the mixture for 30 minutes. This procedure was repeated, whereby a second polymer solution having a viscosity of 300,000 cps was prepared.

The second polymer solution was left at about 0° C. for about one day, and it was then coated onto a glass plate and dried for 30 minutes with hot air of 100° C. The dried polyamide polymer was peeled off from the glass plate and fixed to a pin frame, which was then thermally treated while the temperature was raised at a temperature elevation rate of 2° C./minute in the temperature range of 80° C. to 300° C., thereby obtaining a polyamide-based film having a thickness of 50 μm.

The sequence and amounts of the diamine compound, the dianhydride compound, and the dicarbonyl compound introduced are as described in Table 1 below. The sequence of introduction was described from the top to the bottom of Table 1 from the compound added first, and the amount introduced is described as the number of moles based on 100 moles of the diamine compound.

In Example 2, 1.18 g (0.006 mole) of butane-1,2,3,4-tetra anhydride was used instead of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). In the Comparative Examples, the composition of the polymer was changed as shown in Table 1.

Evaluation Example

The films prepared in the Examples and Comparative Examples were each measured and evaluated for the following properties. The results are shown in Table 1 below.

Evaluation Example 1: Measurement of Modulus

A sample was cut out by at least 10 cm in the direction perpendicular to the main shrinkage direction of the film and by 10 cm in the main shrinkage direction. It was fixed by the clips disposed at an interval of 10 cm in a universal testing machine UTM 5566A of Instron. A stress-strain curve was obtained until the sample was fractured while it was stretched at a speed of 12.5 mm/minute at room temperature. The slope of the load with respect to the initial strain on the stress-strain curve was taken as the modulus (GPa).

Evaluation Example 2: Measurement of Transmittance and Haze

The light transmittance and haze were measured using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo in accordance with the JIS K 7136 standard.

Evaluation Example 3: Measurement of Yellow Index

The yellow index (YI) was measured with a spectrophotometer (UltraScan PRO, Hunter Associates Laboratory) under the conditions of d65 and 100 in accordance with the ASTM-E313 standard.

Evaluation Example 4: Evaluation of Mechanical Properties

The composite hardness (Martens hardness), indentation hardness (Vickers hardness), indentation hardness, indentation modulus, elasticity, and creep of the film samples were measured using FISCHERSCOPE HM2000 of FISCHER. The measurements were carried out in accordance with ISO 14577-1:2015 and 14577-2:2015. Specifically, the film was pressed downward for 20 seconds at a pressure of 30 mN at room temperature with a diamond tip having a shape as shown in FIG. 5, held for 5 seconds, and then elevated upward again.

The nanoindenter tip shown in FIG. 5 may be a modified Vickers indenter. Specifically, in a Vickers indenter having a square base and a pyramid shape where four corners of inclined surfaces meet at a point, one vertex where two inclined surfaces extending in a direction (longitudinal direction) parallel to one side of the bottom surface and facing in the width direction meets one of the inclined surfaces facing in the longitudinal direction may be distanced by 0.5 μm (L) from the other vertex where the two inclined surfaces facing in the width direction meets the other of the inclined surfaces facing in the longitudinal direction. In addition, the tip (to be in contact with a specimen) of the indenter may be rounded so that the radius of curvature (R) is 0.2 μm or less.

TABLE 1 Ex. 1 Ex. 2 C. Ex. 1 C. Ex. 2 C. Ex. 3 C. Ex. 4 C. Ex. 5 Polymerization ratio Diamine TFMB TFMB TFMB TFMB TFMB TFMB TFMB of polyamide-based compound 100 100 100 100 100 100 100 polymer (% by mole) Dianhydride BPDA 3 CBDA 3 — — 6FDA 7 BPDA 40 CBDA 40 compound (% by mole) Dicarbonyl TPC 72 TPC 72 TPC 75 TPC 60 TPC 71 TPC 60 TPC 60 Compound IPC 25 IPC 25 IPC 25 IPC 40 IPC 22 (% by mole) Composite hardness HM 273.7 262.3 253.7 254.6 261.2 270.2 265.3 (Martens hardness; N/mm²) Indentation hardness HV 49.2 49.7 44.7 43.7 45.6 50.74 50.21 (Vickers Hardness; N/mm²) Indentation hardness HIT 520.7 525.7 485.3 462.3 482.7 519.7 531.8 (N/mm²) Indentation modulus EIT 5,050.3 4,899.7 4,882.1 4,878.3 4,953.7 5,087.4 5,000.8 (MPa) Elasticity (%) nIT 62.1 61.9 58.2 57.9 59.3 60.65 59.95 Creep (%) CIT 2.1 2.5 2.6 2.7 2.6 2.5 2.5 First plasticity characteristic value (%) 19.92 19.01 21.85 23.19 22.02 20.46 19.98 Elasticity characteristic value (%) 68.50 68.54 63.99 63.39 65.08 66.85 66.33 Second plasticity characteristic value (%) 405.5 393.2 462.3 486.3 458.4 424.6 416.7 Modulus (GPa) 6.8 6.9 6.80 6.3 6.0 6.2 6.9 Transmittance (%) 89.0 88.2 88.9 89.0 89.0 63.3 81.7 Hz 0.5 0.49 0.4 0.3 0.7 96.4 51.1 YI 2.3 2.02 2.78 1.9 3.1 36.6 27.7

Referring to FIG. 1, in the films having a first plastic characteristic value, a second plastic characteristic value, and/or an elastic characteristic value according to the embodiments, its mechanical properties such as resistance to indentation (hardness), restoring force (elasticity), and tensile modulus were enhanced, as well as excellent its optical properties such as transmittance, haze, and yellow index were excellent.

EXPLANATION OF REFERENCE NUMERALS

  100: polyamide-based film 101: first side 102: second side 200: functional layer 300: cover window 400: display unit 500: adhesive layer 

1. A polyamide-based film, which comprises a polyamide-based polymer, wherein the plasticity characteristic value represented by the following Equation 1 is 18.45 to 19.97%: Plasticity characteristic value (%)=(100−nIT)/(HIT/HM)  [Equation 1] in Equation 1, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm²), and HM is the composite hardness (N/mm²).
 2. The polyamide-based film of claim 1, wherein the polyamide-based polymer comprises an amide-based repeat unit and an imide-based repeat unit at a molar ratio of 100:0 to 90:10.
 3. The polyamide-based film of claim 2, wherein the amide-based repeat unit comprises a first amide-based repeat unit derived from a first dicarbonyl compound and a second amide-based repeat unit derived from a second dicarbonyl compound, and the angle between the two carbonyl groups contained in the first dicarbonyl compound is greater than the angle between the two carbonyl groups contained in the second dicarbonyl compound.
 4. The polyamide-based film of claim 3, wherein the molar ratio of the repeat unit derived from the first dicarbonyl compound and the repeat unit derived from the second dicarbonyl compound is 25:75 to 75:25.
 5. The polyamide-based film of claim 1, which, based on a thickness of the film of 50 μm, has a modulus of 6 GPa or more, a transmittance of 80% or more, a haze of 1% or less, and a yellow index or 3 or less.
 6. The polyamide-based film of claim 1, wherein the elasticity characteristic value represented by the following Equation 2 is 67 to 69.5%: Elasticity characteristic value (%)=nIT/{EIT/(EIT+HIT)}  [Equation 2] in Equation 2, nIT, EIT, and HIT are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).
 7. The polyamide-based film of claim 1, wherein the second plasticity characteristic value represented by the following Equation 3 is 384 to 416%: Second plasticity characteristic value (%)=(1−nIT)/{HIT/(EIT+HIT)}  [Equation 3] in Equation 3, nIT, EIT, and HIT are measured according to the ISO 14577 standard through a nanoindenter, nIT is the elasticity (%), EIT is the indentation modulus (GPa), and HIT is the indentation hardness (N/mm²).
 8. A process for preparing the polyamide-based film of claim 1, which comprises polymerizing a diamine compound and a dicarbonyl compound in an organic solvent to prepare a solution comprising a polyamide-based polymer; casting the polymer solution to prepare a gel sheet; and thermally treating the gel sheet.
 9. The process for preparing the polyamide-based film according to claim 8, wherein the step of polymerizing the diamine compound and the dicarbonyl compound comprises: reacting the diamine compound and the first dicarbonyl compound with the second dicarbonyl compound to form a first polymer solution having a viscosity of 10,000 to 150,000 cps; and reacting the first polymer solution with an additional amount of the second dicarbonyl compound to form a second polymer solution having a viscosity of 200,000 to 350,000 cps.
 10. The process for preparing the polyamide-based film according to claim 9, wherein, in the step of forming the first polymer solution, the first dicarbonyl compound and the second dicarbonyl compound are sequentially reacted with the diamine compound.
 11. The process for preparing the polyamide-based film according to claim 9, wherein the step of forming the first polymer solution further comprises reacting the diamine compound and the dianhydride compound before the dicarbonyl compounds are added.
 12. A cover window for a display device, which comprises a polyamide-based film and a functional layer, wherein the polyamide-based film has a plasticity characteristic value represented by the following Equation 1 of 18.45 to 19.97%: Plasticity characteristic value (%)=(100−nIT)/(HIT/HM)  [Equation 1] in Equation 1, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm²), and HM is the composite hardness (N/mm²).
 13. A display device, which comprises a display unit; and a cover window disposed on the display unit, wherein the cover window comprises a polyamide-based film and a functional layer, and the polyamide-based film has a plasticity characteristic value represented by the following Equation 1 of 18.45 to 19.97%: Plasticity characteristic value (%)=(100−nIT)/(HIT/HM)  [Equation 1] in Equation 1, nIT, HIT, and HM are measured according to the ISO 14577 standard with a nanoindenter, nIT is the elasticity (%), HIT is the indentation hardness (N/mm²), and HM is the composite hardness (N/mm²). 